Reaction-diffusion Coupling Research Articles

Before programs: The physical origination of multicellular forms

By examining the formative role of physical processes in modern-day developmental systems, we infer that although such determinants are subject to constraints and rarely act in a "pure" fashion, they are identical to processes generic to all viscoelastic, chemically excitable media, non-living as well as living. The processes considered are free diffusion, immiscible liquid behavior, oscillation and multistability of chemical state, reaction-diffusion coupling and mechanochemical responsivity. We suggest that such processes had freer reign at early stages in the history of multicellular life, when less evolution had occurred of genetic mechanisms for stabilization and entrenchment of functionally successful morphologies. From this we devise a hypothetical scenario for pattern formation and morphogenesis in the earliest metazoa. We show that the expected morphologies that would arise during this relatively unconstrained "physical" stage of evolution correspond to the hollow, multilayered and segmented morphotypes seen in the gastrulation stage embryos of modern-day metazoa as well as in Ediacaran fossil deposits of approximately 600 Ma. We suggest several ways in which organisms that were originally formed by predominantly physical mechanisms could have evolved genetic mechanisms to perpetuate their morphologies.

Source of reaction-diffusion coupling in confined systems due to temperature inhomogeneities.

Diffusion is often accompanied by a reaction or sorption which can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported with a hot zone presumed to be created by a reaction. Our simulations show that localized hot regions can alter both the kinetic and transport properties. Further, enhancement of the diffusion constant is greater for larger barrier height, a surprising result of considerable significance to many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of hot zone in addition to that which normally exists via concentration.

Generic physical mechanisms of morphogenesis and pattern formation as determinants in the evolution of multicellular organization

Early embryos of metazoan species are subject to the same set of physical forces and interactions as any small parcels of semi-solid material, living or nonliving. It is proposed that such “generic” properties of embryonic tissues have played a major role in the evolution of biological form and pattern by providing an array of morphological templates, during the early stages of metazoan phylogeny, upon which natural selection could act. The generic physical mechanisms considered include sedimentation, diffusion, and reaction-diffusion coupling, all of which can give rise to chemical nonuniformities (including periodic patterns) in eggs and small multicellular aggregates, and differential adhesion, which can lead to the formation of boundaries of non-mixing between adjacent cell populations. Generic mechanisms that produce chemical patterns, acting in concern with the capacity of cells to modulate their adhesivity (presumed to be a primitive, defining property of metazoa), could lead to multilayered gastrulae of various types, segmental organization, and many of the other distinguishing characteristics of extant and extinct metazoan body plans. Similar generic mechanisms, acting on small tissue primordia during and subsequent to the establishment of the major body plans, could have given rise to the forms of organs, such as the vertebrate limbs. Generic physical processes acting on a single system of cells and cell products can often produce a widely divergent set of morphological phenotypes, and these are proposed to be the raw material of the evolution of form. The establishment of any ecologically successful form by these mechanisms will be followed, under this hypothesis, by a period of genetic evolution, in which the recruitment of gene products to produce the “generically templated” morphologies by redundant pathways would be favoured by intense selection, leading to extensive genetic change with little impact on the fossil record. In this view, the stabilizing and reinforcing functions of natural selection are more important than its ability to effect incremental change in morphology. Aspects of evolution which are problematic from the standard neo-Darwinian viewpoint, or not considered within that framework, but which follow in a straightforward fashion from the view presented here, include the beginnings of an understanding of why organisms have the structure and appearance they’ do, why homoplasy (the recurrent evolution of certain forms) is so prevalent, why evolution has the tempo and mode it does (“punctuated equilibrium”), and why a “rapid” burst of morphological evolution occurred so soon after the origin of the metazoa.

‘Generic’ physical mechanisms of morphogenesis and pattern formation

The role of 'generic' physical mechanisms in morphogenesis and pattern formation of tissues is considered. Generic mechanisms are defined as those physical processes that are broadly applicable to living and non-living systems, such as adhesion, surface tension and gravitational effects, viscosity, phase separation, convection and reaction-diffusion coupling. They are contrasted with 'genetic' mechanisms, a term reserved for highly evolved, machine-like, biomolecular processes. Generic mechanisms acting upon living tissues are capable of giving rise to morphogenetic rearrangements of cytoplasmic, tissue and extracellular matrix components, sometimes leading to 'microfingers', and to chemical waves or stripes. We suggest that many morphogenetic and patterning effects are the inevitable outcome of recognized physical properties of tissues, and that generic physical mechanisms that act on these properties are complementary to, and interdependent with genetic mechanisms. We also suggest that major morphological reorganizations in phylogenetic lineages may arise by the action of generic physical mechanisms on developing embryos. Subsequent evolution of genetic mechanisms could stabilize and refine developmental outcomes originally guided by generic effects.

Periodic forcing of the oscillatory CO oxidation on Pt single-crystal surfaces

The kinetic oscillations occurring in the oxidation of carbon monoxide on Pt(110) and Pt(100) single-crystal surfaces at low pressure under isothermal conditions exhibit, despite a similar underlying mechanism, some pronounced differences in phenomenology between the two surfaces, which become most clearly evident in studies of the system response to external periodic perturbations. These differences can be rationalized in terms of different types of spatial self-organization on the two surfaces in question, namely, reaction–diffusion coupling on Pt(100) and feedback through the gas phase for Pt(110).